I zoomed in on each soldier's face and don't see any black soldiers. One of the drummers in the front rank has a somewhat darker complexion, but when zooming in he is definitely Caucasian.

There are some citizens hanging around the rear of the company, but their features are not clear.

At any rate, the original roster of the Hempstead Rifles is extant, and all of the soldiers are white residents of Hempstead County and vicinity.